The quad antenna, also called cubical quad, is a directional wire antenna. The basic element is a radiator 11 constructed in the manner of a square, see FIG. 1. The side length 9 is .lambda./4. The entire wire length is thus 1.lambda. (wavelength). At a distance A of 0.15 - 0.20 .lambda. a second, identical wire quadrangle is disposed, which is somewhat larger dimensions, about 1.03 - 1.05 .lambda. , this latter component acting as a reflector 21. This increase in length of the reflector element 21 is frequently produced, while maintaining the same dimensions of the square as in case of the associated radiator, by means of an inserted extension piece, e.g. a wire bracket maintained at the same distance, the stub 8, or in the form of a wire section wound into a coil. A quad antenna wherein the reflector element has its full length is called, in contrast thereto, a "full-phased reflector." The quad antennas radiate in a horizontally polarized manner if fed at the base point, i.e. at the point of the radiator closest to the ground (earth). In contrast thereto, if fed laterally, they would radiate in a vertically polarized manner.
To obtain a further gain in the efficiency of the antenna, it is also possible, in addition to the provision of reflectors, to employ so-called director elements. While the reflectors are longer than the radiators and thereby act on the radiator by an inductive phase shift, the directors are shorter than the radiator and effect a capacitive phase shift. The power gain attainable by these unfed elements, namely the reflectors and directors, depends on the spacing with respect to the radiator. As already set forth above, the reflector distance to the radiator ranges between 0.10 and 0.25 .lambda., while the spacing of the director to the radiator is between 0.10 and 0.15 .lambda..
Moreover, the distance between the radiator and the reflector and director, respectively, is decisive for the base impedance of the antenna, for the antenna gain in the radiation direction, and for the forward-backward ratio of the antenna. Desirable as the base impedance is 50-60 ohms, i.e. a value maximally corresponding to the value of the resistance of the feeder cable 7, i.e. a coaxial cable connecting transmitter and antenna. The smaller the distance between the elements, radiator, director, reflector, the lower the base impedance. The feed 7 of the antenna can be direct or indirect, depending on the respective requirements, wherein the indirect feeding is carried out by way of a balun or via the so-called gamma adapter, that is a frequency-linked adapting element for asymmetrical feeder cables with adjustable resistance. Feeding by means of a balun, though, has the purpose of obtaining symmetry, in order to prevent thereby the so-called squinting of the antenna.
The quad antenna can be constructed as a single-band or multiband antenna with two, three, four, or even five elements; in other words, one or more directors are arranged additionally to the radiator and the reflector. Mechanical problems are encountered during the construction of the antenna, in that the elements, i.e. two, three, or even four or five wire squares of large dimensions must be arranged in series at accurate spacings. The entire arrangement must be lightweight, rugged in its construction, and erectable in a mechanically simple and safe fashion. If the quad antenna is furthermore to be usable for more than one band, i.e. for several frequencies, then several wire squares of varying sizes must be mounted as the elements. Conventional structures of a quad antenna utilize a braced cross with spreaders 3 as shown in FIG. 1, attached to a horizontal mounting pipe 10. For each element, one braced cross is required; several taut wire squares for various bands (frequencies) can be provided at one braced cross. The elements, i.e. the wire quadrangles, are attached to the braced cross so that they are insulated; it is desirable to employ, if at all possible, no metal for the entire mounting structure, except for the feeder cable in the antenna. In case of side lengths of the wire squares of 5 meters and more (for example 20 m, band), considerable mechanical difficulties result, since the antenna must be assembled on the ground, where it is level, and then is attached to the masthead above the ground.
Another conventional construction of a quad antenna operates with spreaders in spider form, i.e. without a mounting pipe, wherein all element spreaders meet in the center of the cube. Also in case of this antenna, the preliminary assembly takes place on level ground, and considerable difficulties are encountered in mounting the antenna in the assembled condition to the masthead, inasmuch as it must be attached in its entirety from above onto the masthead so as not to destroy the wire squares, i.e. the elements. High masts are required for the spider quad antenna as well as for the braced cross quad antennas, ensuring the necessary distance from the ground and a free rotation. A subsequent tuning of the element is impossible without removing the antenna from the mast.
A modification of a quad antenna is the delta loop antenna operating with a horizontal mounting pipe and based on this triangular element. The delta loop antenna is susceptible to wind forces and cannot be used as a multiband antenna, utilizing the same spreaders for all bands, since the spreaders here function simultaneously as the element.